The expansion of diagnostic categories in the late nineteenth century to include behaviors not previously interpreted as medical problems raised questions about the validity of defining behaviors as disease and the role of individual responsibility for these behaviors. One instance of this medicalization of behavior was the elaboration of "sexual inversion" as a new disease in American medical writings of the period. In searching for a defining characteristic of diverse cases, clinicians described sexuality as a fundamental aspect of being rather than simply an aspect of behavior, and created an identity for their patients that altered the way they thought about themselves and were viewed by society. Although the medical diagnosis later became a central feature in the social oppression of homosexuals, it offered the possibility of construing behaviors in a new way.